CSLB C-10 Electrical ContractorSafety and TestingHard
A qualified worker is using a digital multimeter (DMM) to measure AC voltage on a live circuit. Before making any measurements, what essential safety step must be performed related to the DMM itself?
- ACalibrate the DMM against a known DC voltage source.
- BEnsure the DMM's fuses are bypassed for higher current readings.
- CVerify the DMM's battery is fully charged.
- DConfirm the DMM is set to the correct function (AC Volts) and range (if not auto-ranging).
Show answer & explanationAnswer & explanation
Correct answer: D. Confirm the DMM is set to the correct function (AC Volts) and range (if not auto-ranging).
Before using a DMM on a live circuit, it is critically important to set the meter to the correct function (e.g., AC Volts, DC Volts, Amps) and an appropriate range. Setting it to an incorrect function, such as Ohms or Amps, on a live voltage source can damage the meter, blow fuses, and create a hazardous arc flash or shock incident.
Why the other options are wrong
- A. Calibration is a periodic maintenance task, not an immediate pre-measurement safety step for every use.
- B. Bypassing fuses is extremely dangerous and compromises the meter's safety features.
- C. While a charged battery is important for accurate readings, it's not the most immediate safety-critical step before contacting a live circuit.
DMM Pre-Use Safety Check
Before connecting a Digital Multimeter (DMM) to a live circuit, always ensure it is set to the correct measurement function (e.g., AC Volts) and an appropriate range (if not auto-ranging) to prevent meter damage and ensure user safety.
- Incorrect settings can cause meter damage or electrical hazards.
- Always select a range higher than the expected voltage initially.
- Check lead connections to the correct input jacks.
- Part of a comprehensive 'prove-before-prove-after' testing sequence.
Memory trick: Function First, Range Right, Leads Last, Live Later!